Field of the Invention
The class of aluminum oxide reagents known as “transition aluminas” play commercially important roles as catalysts or catalyst supports in many chemical processes, including the cracking, hydrocracking and hydrodesulfurization of petroleum, the steam reforming of hydrocarbon feed stocks ranging from natural gas to heavy naphthas to produce hydrogen, the synthesis of ammonia, and the control automobile exhaust emissions, to name a few. Transition aluminas also are used extensively as absorbents.
The usefulness of transition aluminas in catalysis and adsorption processes can be traced to a combination of favorable textural properties (i.e., relatively high surface areas and porosity) and surface chemical properties that can be either acidic or basic depending in part on the transition alumina structure and on the degree of hydration and hydroxylation of the surface. Structurally, all transition aluminas are disordered crystalline phases. Although the oxygen atoms are arranged in regularly ordered close packed arrays, the aluminum atoms adopt different ways of occupying the tetrahedral and octahedral interstacies within the oxygen lattice. Variations in the relative placement of aluminum ions in the tetrahedral and octahedral positions leads to different phases that can be distinguished by NMR techniques and by x-ray diffraction and other scattering methods. At least seven different transition alumina phases have been described, namely, chi, kappa, rho, eta, gamma, delta, and theta (Wefers, K. and Misra, C., Oxides and Hydroxides of Aluminum, Alcoa Technical Paper No. 19, Revised, Alcoa Laboratories, 1987).
Transition aluminas are formed through the thermal dehydration and dehydroxylation of aluminum trihydroxides (e.g., gibbsite or bayerite) or aluminum oxyhydroxides (e.g., boehmite, diaspore). Collectively, the hydroxides and oxyhydroxides of aluminum are called aluminum hydrates or hydrated aluminas, although they have very different formulas corresponding to Al(OH)3 and AlO(OH), respectively. The thermal dehydration of gibbsite can lead to the formation of chi, kappa, rho, eta or theta transition aluminas, depending on the heating rate, the dwell temperature and the atmosphere in contact with the solid phase. The thermal dehydration of boehmite can afford gamma, eta, delta, or theta phases, depending on the conditions of dehydration and the particle size and degree of crystallinity of the starting boehmite (Wefers, K. and Misra, C., Oxides and Hydroxides of Aluminum, Alcoa Technical Paper No. 19, Revised, Alcoa Laboratories, 1987. Pseudoboehmite, a poorly ordered form of boehmite with a small primary particle size, is often a preferred precursor to transition aluminas, because it typically affords derivatives with relatively high surface areas and pore volumes. Boehmite and pseudoboehmite are useful aluminas in their own right, particularly when they are in high surface area form. For instance, Rehyrazal™ is a high surface area boehmite that is used extensively as a vaccine adjuvant (www.reheis.com; Gupta, R. K., Advanced Drug Delivery Reviews, 32 155-172 (1998)).
All transition aluminas will form the structurally stable and comparatively inert aluminum oxide known as alpha alumina when heated to a temperature above about 1000° C. Because transition aluminas are formed through thermal dehydration processes, they are sometimes called “activated aluminas”. However, the term “transition aluminas” is more appropriate, because these phases are encountered as intermediates along the thermal pathways that transform hydrated aluminas to alpha alumina.
Among the transition aluminas mentioned above, those derived from the thermal dehydration of boehmite and pseudoboehmite, particularly gamma and eta, are often preferred for catalytic and adsorption applications. Gamma alumina is formed from well ordered boehmite at a temperature above about 400 to 450° C. depending on the particle size. Pseudoboehmite, a disordered form of boehmite containing an amorphous alumina fraction, can be transformed to eta alumina upon dehydration. Gamma alumina formed from course grained boehmite may be transformed to delta alumina at about 800° C. Both eta and delta aluminas transform to theta alumina at temperatures above about 800-900° C. depending on particle size. Finally, theta alumina transforms to alpha alumina above about 1000° C.
Recently reported studies indicate that these transition alumina phases can be mixtures of transition phases with one transition alumina phase being dominant. But the purity of the transition alumina phase is not the limiting factor in determining the performance properties in catalysis and adsorption. Normally, it is the textural properties (i.e., the pore size, pore volume, and surface area), along with the surface chemical properties, that determine the performance properties of a transition alumina in catalysis and adsorption. As noted earlier, the phase and hydration state of the surface determines the surface properties. However, the textural properties are determined by the fundamental (primary) particle size of the alumina, as well as the aggregated particle size. By optimizing the textural properties, one may expect to greatly improve the performance properties of a transition alumina derived from boehmite. The surface areas of most commercially available gamma aluminas, for example, typically have a BET surface area<250 m2/g and a pore volume<0.50 cc/g. Thus, there is a need to develop transition alumina phases with substantially improved textural properties in order to achieve improved performance in catalysis and adsorption.
It has been recognized recently that the surface area and porosity of an alumina can be substantially increased by forming a mesostructure through supramolecular assembly pathways (Bagshaw, S. A.; Pinnavaia, T. J., Angew. Chem. Intern. Ed. Engl. 1996, 35, 1102-1105; Pinnavaia, T. J.; Bagshaw, S. A., U.S. Pat. No. 6,027,706). In this approach a surfactant is used to direct the formation of a mesostructure with walls comprised of the alumina. Removing the surfactant by solvent extraction or by calcination generated a mesostructured alumina. The formation of a mesostructure was indicated by the presence of at least one low angle refection in the x-ray diffraction patterns of the as made alumina-surfactant composition and the final surfactant-free alumina. The low angle diffraction peak corresponded to a pore to pore correlation distance of at least 2.0 nm. Several examples of similar mesostructured aluminas have been reported more recently (Davis et al., Chem. Mater. 1996, 8, 1451; Gabelica et al., Microporous Mesoporous Mater. 2000, 35-36, 597; Cabrera et al. Adv. Mater. 1999, 11, 379). For all of these previously reported mesostructured aluminas, however, the walls of the mesostructure were amorphous. That is, neither the oxygen atoms nor the aluminum atoms were arranged on lattice points, as indicated by the absence of Bragg reflections in the wide angle region of the diffraction patterns. Consequently, these reported mesostructured aluminas can be described as being mesostructured alumina gels. They have limited stability under hydrothermal conditions. Also, these mesostructured aluminas with atomically amorphous framework walls lacked the desired surface acidity and basicity characteristic of an atomically ordered transition alumina, thus limiting their usefulness in chemical catalysis and adsorption. Thus, there is a need to form mesostructured forms of transition aluminas with atomically ordered pore walls, as well as mesostructured forms of hydrated aluminas which serve as precursors to transitions aluminas.